What is the menstrual cycle… exactly?
Warning: Science heavy.
The menstrual cycle is an intricate series of hormonal interactions within the endocrine system, whose purpose is to prepare the female body for potential pregnancy. The physiological process is characterised by predictable cyclical changes in the secretion of hormones, which regulates the growth, maturation, and release of an egg and prepares the uterus for conception.
That is a mouthful, I know! Stay with me you will be amazed by your own body, I promise!
The cycle begins at menstruation, where low levels of oestrogen and progesterone lead to uterine lining shedding, the bleed, which is triggered by a decrease in both hormones from the previous cycle. At the same time, the pituitary gland (see previous ‘menstrual cycle hormones’ blog), begins to secrete follicle-stimulating hormone (FSH), which facilitates the growth of several ovarian follicles (small functional fluid-filled sac), which each contain a potential ovum (egg). This part of the cycle is labelled the follicular phase or pre-ovulatory phase.
As the follicular phase progresses, the growing follicles secrete increasing amounts of oestrogen, which exerts negative feedback on the pituitary gland to moderate its secretion of FSH, ensuring that typically only one follicle continues to maturity. This rise in oestrogen also stimulates the proliferation of the uterine lining, preparing it for potential implantation.
A feedback system in the body is basically a loop of circulating information that triggers either a less button (negative feedback) or more button (positive feedback), to decrease or increase hormone levels.
The hormonal peak of the follicular phase is marked by a dramatic surge in luteinizing hormone (LH) secretion, which is induced by a positive feedback loop, where the high levels of oestrogen from the dominant follicle, stimulate the pituitary gland to release huge quantities of LH. This LH flush triggers ovulation, which is the release of the now mature egg, from the follicle, into the fallopian tube. Ovulation occurs on approximately mid cycle on day 14ish.
Following ovulation, the luteal phase (second half of the cycle) begins as the ruptured follicle transforms into the corpus luteum (a temporary cluster of cells), which secretes high levels of progesterone and oestrogen. Progesterone is a big player in this phase, thickening the uterine lining and making it more glandular and receptive to a potentially fertilised egg. This hormonal environment is maintained by the corpus luteum, which relies on continued LH support for its survival and function.
In the absence of fertilisation, the corpus luteum eventually degenerates, resulting in a sharp decline of progesterone and oestrogen. This hormonal withdrawal causes the uterine lining to shed, initiating the start of a new menstrual phase. The drop in oestrogen and progesterone removes the negative feedback on the hypothalamus and pituitary; and so, after 28ish days the cycle to begins again!
You made it!
It’s ok if you don’t absorb this information straight away, you can come back and read it 20 times if you need to. If you just grasp the main take aways, that’s ok too! Learning the workings of your female body, is not easy on the mind but it is deeply empowering and humbling, which is the perfect mindset to cultivate body love and self-knowledge!
Keisha xx
